This invention relates to a fastening system. It serves for attaching a structural component to a carrier material.
It is been known already to insert a plug into a bore of a carrier material and to locate it therein by friction or securing it in place. In such a case, the plug has a bore into which a screw is rotated in order to fasten the structural component to the carrier material. The plug may then be located in the carrier material by forcing it in or latching it in by hinged feet or widening it by means of the screw. In known plastic plugs, threads are formed in the bore by rotating the screw in, e.g. by thread-molding or thread-cutting. For this purpose, the screw has a flank angle, which is comparatively acute as compared to screws for metal sheeting or metric-thread screws and typically is 30xc2x0. In addition, such screws have a pitch which is large as compared to screws for metal sheeting or metric-thread screws and may be 2.24 mm, for example.
The aforementioned fastening may be employed particularly for flat carrier materials which have a through-bore to receive a plug. Such a fastening is used especially in automobile manufacture, e.g. for attaching guide strips or panelings to the body.
In addition, fastenings for structural components to flat carrier materials are known already, which include a metallic screw and a metallic clip. The metallic screw typically has a relatively large flank angle of about 60xc2x0 and a relatively small pitch of 1.6 mm, for example. The metallic clip has a through-bore which traverses either clip bow. One clip bow has formed in it a thread. The metallic clip is slid onto the edge of a carrier material so that its through-bore is aligned with a through-bore of the carrier material. The metallic screw is then rotated into the flush through-bores with its thread interacting with the thread of the metallic clip until the metallic screw is secured in place with the metallic clip and the carrier structural component which is received thereby. The structural component is secured on a through-bore between the screw head and the metallic clip. The threaded joint is self-locking.
Fastenings of this type are also employed specifically in automobile manufacture when structural components require to be attached in the marginal area of metal sheetings, e.g. panelings to wheel housings or safety bumpers to the body.
A drawback is that, apart from the plastic plug and the metallic clip, different screws need to be manufactured, kept in stock, and handled for these different fastenings which are employed in the same areas.
Based on this fact, it is an object of the invention to provide a less expensive fastening, system for fixing structural components to carrier materials.
The inventive fastening system includes
a screw which comprises a screw head and on a screw shank a thread which with respect to the pitch and the flank angle is matched to the formation of a thread in a bore of a plastic material, and at least one axially extending deepening on the thread root, and
a plastic material with a bore into which the screw may be threaded whilst forming a thread and/or
clip with two clip bows and a clip back which connects these and which comprises a through-bore traversing the clip bows, with a thread for rotating in the screw and at least one resilient lug aligned radially to the axis of the through-bore for latching into the at least one deepening of the screw when this is rotated into the through-bore of the clip.
The fastening system comprises a xe2x80x9cplastic-tailoredxe2x80x9d screw. This means that the screw is configured as to its pitch and flank angle in such a way that it is capable of forming an appropriate thread in a bore of a plastic material. At this point, the pitch must not be dimensioned so small and the flank angle must not be dimensioned so large that the formation of a thread in the bore can no longer be achieved at an acceptable expenditure and the load-carrying ability of the threaded joint is too low. On the other hand, the pitch must not be chosen so large to prevent the engaging threads from being self-locking and the flank angle chosen must not be chosen so small that the load-carrying ability of the thread flank becomes insufficient.
Preferably, the thread of the screw has a flank angle of about 20 to 40xc2x0, especially about 30xc2x0. Preferably, the thread of the screw further has a pitch of from 1.5 to 3.5 mm, specifically 2.2 mm (cf. the embodiment), which particularly depends on the measure of the outside diameter.
However, the ranges given may be exceeded or remain below the mark because shaping a plastic-tailored thread may be dependent on influencing parameters such as the surface finish of the screw and the substance of the plastic material, the dimensions of the screw and the bore, and the speed of the screw-driver for rotating in the screw, which may vary very much from one case to another.
The design of the screw ensures that structural components can be fixed to plastic materials. This may concern, in particular, a plug made of plastic or carrier materials in plastic or plastic portions of carrier materials thereof. In particular, it may concern a plastic dome of a carrier material which may be of a different material. The carrier materials may be both thick and flat.
Furthermore, it is possible to fix a structural component to a flat carrier material or a flat portion of a carrier material by means of a clip. It has a thread which is matched to the thread of the plastic-tailored screw, i.e. it has a pitch larger than that of a conventional metallic clip. This screwed joint has less self-locking properties. However, it is ensured that the clip be locked by the fact that the clip latches its at least one resilient lug into the at least one axially extending deepening of the screw when this one has been rotated into the clip. It is understood here that the at least one lug may be latched several times into the at least one deepening while the screw is rotated in, and may be unlatched again therefrom. Preferably, there are several deepenings to make arresting possible in various angular positions of the screw. Also, several lugs may be uniformly spaced around the through-bore of the clip in order to uniformly load the screw at the circumference.
The deepening extends in the axial direction of the screw, which deepening may also run in a helical shape. What is essential is that it has a pitch which at least differs from that of the screw thread. Preferably, however, the at least one deepening extends parallel to the screw shank. Since the screw thread has a relatively large pitch it may have a thread root which may be relatively wide and on which at least one deepening may be accommodated.
The at least one deepening may be of a symmetrical (e.g. a U or V-shaped) or an asymmetrical (e.g. a saw-tooth shaped) cross-section. The latter offers a possibility, while the screw is rotated into the thread of the clip, to make it easier for the lug to exit from the deepening by appropriately inclining one flank thereof and, if there is a load in the opposed direction, to make it more difficult for the lug to exit from the deepening by causing the lug to abut against a correspondingly steep flank.
The clip preferably is designed as a spring clip where a spring bow may have an angled tab to make it easier to slide the spring clip onto a flat material. In addition, the clip may have one or more barbs at the inside to secure the clip to a structural component in a pre-assembled position.
The at least one lug may have an end portion tapering towards the free end, which makes it more convenient to rotate in the screw and to precisely lock it in a determined position. Preferably, the at least one lug is bent out of the flat material of the clip. Although the material of the clip may be a metal, particularly a metal sheeting, a plastic may be considered as well.